Research

The group has moved to the Department of Microsystems Engineering at the University of Freiburg.

Our research revolves around computer modeling of mechanical and chemical properties of material interfaces and their interaction with fluids and other solids. We have a particular focus on tribological phenomena (contact, adhesion, friction, lubrication, wear) that naturally occur at interfaces. Such processes are important in macro- and microsystems and their control is decisive for the lifetime of a device. For example in miniaturized components that have a high surface to volume ratio, interfacial processes can entirely dominate mechanical behavior: At small scales, strength is determined by surface and not bulk defects, the flow of liquids through nanochannels can be controlled by surface topography and chemistry and surface forces such as adhesion and friction can overcome body forces and lead to stiction. Common to these phenomena is the interplay of local chemistry, long-ranged interaction (such as elasticity) and geometrical disorder (such as surface roughness). Models at atomic, mesoscopic and macroscopic scales are therefore required for their understanding.

Multiasperity contact and adhesion. Contact, friction and wear of natural and engineered surfaces cannot be understood without consideration of surface roughness. Roughness limits the area of intimate atomic contact to isolated areas where summits (asperities) meet and hence where the process zone develops. We use large scale contact mechanical calculations at mesoscopic and continuum sclaes to study the influence of surface topography on macroscopic properties such as contact stiffness, adhesion and friction.

Single-asperity contact and adhesion. Spherical objects, such as tips on an atomic-force microscope (AFM), are often used as simplifying models for the contact of a single asperity in a rough contact. AFM experiments are carried out at scales that are in principle accessible by molecular simulations. We carry out such calculations to aid the interpretation of experimental AFM friction, topography and adhesion data.

Shear-induced melting and solid state amorphization. Mechanically sheared systems can undergo structural transitions. Examples are colloidal crystals that shear-melt or the solid state amorphization of diamond or silicon during tribological loading. Such disordered phases often appear on tribologically loaded interface and it is believe that they constitute a form of solid lubricant. We use molecular dynamics calculations of simple model materials to study the formation of glasses during mechanical shear.

Mechanics of amorphous materials. Motivated by the above, we are also interested in the mechanical behavior of traditional bulk amorphous materials. These deform in localized rearrangements of regions of ~100 atoms, so called shear transformations, which coalesce into shear bands on long time scales. We study the deformation of network and metallic glasses in molecular dynamics calculations.

Lubrication. Liquids are ubiquitously used to reduce friction and wear in machinery. We use molecular dynamics calculation to study simple lubricants in confined dimensions and the effect of surface topography and surface chemistry on their flow. We are particularly interested in squeeze out of lubricant and the transition from lubricated to solid contact under boundary lubrication conditions.

The course introduces contact mechanics of smooth and rough surface for non-adhesive and adhesive interfacial conditions. There will a computer lab held in parallel to the lecture that teaches numerical approaches to contact mechanical problems.

Introduction: contact area and stiffness

Theory of the elastic half-space

Contact of nonadhesive spheres: Hertz theory

Physics and chemistry of adhesive interactions at interfaces

Contact of adhesive spheres: theories of Johnson-Kendall-Roberts, Derjaguin-Muller-Toporov and Maugis-Dugdale

Software

We are actively developing open-source academic simulation tools.

Atomistica

Atomistica is a library of interatomic potentials for use with LAMMPS and ASE, two widely used simulation environments for atomic-scale simulations such as molecular dynamics. Atomistica is developed in collaboration with the group of Michael Moseler at Fraunhofer IWM.

matscipy

matscipy is a library of utility function for fracture and contact mechanical problems based on ASE. matscipy is developed in collaboration with James Kermode at Warwick University.

USER-GFMD

USER-GFMD is a plugin for the widely used molecular dynamics code LAMMPS that implements Green's function molecular dynamics. USER-GFMD is developed with the group of Mark Robbins at Johns Hopkins University.

NetCDF trajectory file format

We are maintaining input and ouput modules for molecular dynamics trajectory files in the AMBER NetCDF trajectory format. Modules are available for ASE, LAMMPS and Ovito.

SYMPLER

SYMPLER is an SPH/DPD simulation environment with a flexible handling of interaction laws. Interaction laws are entered in the form of compact expression in the input file rather than being hard-wired into the code. SYMPLER is maintained by David Kauzlaric.